There is a continuing demand to enhance the functioning of the electrical components and mechanical components associated with the transmission of electromagnetic signals from a location at great depths in the earth to the earth's surface. Such communication is used in various types of drilling operations, such as measuring while drilling (MWD) and/or logging while drilling (LWD). Other situations, which warrant communication with the earth surface, is during the inspection and evaluation of bore holes. Such communication is particularly important when carrying out directional drilling such as under river beds, subways, unusual earth formations and tapping oil reservoirs. It is important at all times to know precisely the location of the drill bit. A significant effort has been made to develop electrical instruments which are capable of transmitting signals at the drill face or inspection face back to the earth's surface. Some systems involve the use of the lower portion of the drill string as an antenna for purposes of transmitting via electromagnetic waves, information to the earth's surface, such as described in U.S. Pat. No. 5,394,141. Such system does not involve the use of an isolation subassembly in the drill string.
Various types of devices which are mounted on the outside of the drill string for monitoring surrounding conditions and/or used in communication are described, for example, in U.S. Pat. No. 4,684,946 to Geoservices and U.S. Pat. No. 5,467,832 to Schlumberger Technology Corporation. The problem with mounting communication devices and sensing devices on the exterior of the drill string is that particularly with directional drilling the exterior devices are damaged by striking the formations about the bore hole.
In order to enhance communication with the earth's surface, it is preferred to electrically isolate drill string components so that electromagnetic signals can be developed for data telemetry. This is achieved by a subassembly connector which electrically isolates adjacent drill string components so that the isolated components provide the two terminals of an antenna to which an alternating current is applied in developing the electromagnetic signal for transmission to the earth's surface. Examples of such connectors are described in U.S. Pat. No. 5,138,313 to Haliburton Company; U.S. Pat. No. 5,163,714 to Geoservice and Canadian patent application 2,151,525 to McAllister Petroleum Services, Ltd. The various types of subassemblies provide for electrical isolation which are particularly useful in bore hole inspection, but may be subject to failure when used, for example, in directional drilling. It has been found that the drill string, and in particular the subassembly connector, is subjected to extreme torsional compression, tension, and bending moments during directional drilling. Such extreme forces can result in connector failure, usually at the weakest point in the subassembly. The connectors of these patents and patent application may fail due to overstressing and possibly break up at their weakest point. The connectors may even fail to the extent, particularly those of the US patents, that retrieval of the drill bit and other components below the subassembly cannot be achieved.
In accordance with an object of an aspect of this invention, a subassembly electrical isolation connector is provided which has enhanced strength characteristics for resisting drilling forces, particularly during directional drilling, ensures electrical isolation between adjacent drill string components and in the event of connector failure, ensures retrieval of drill string components below the subassembly.